In recent years, there is an increasing demand for tomatoes and especially high-sugar content tomatoes have been becoming popular. In order to achieve high sugar contents of tomatoes, a load of water stress (restriction of the amount of water supplied to or absorbed by tomato plants) during cultivation is widely employed. There are various known cultivation methods, equipment, and the like for achieving high sugar contents of tomatoes by the load of water stress (e.g., Patent Literature 1 to 2). Treatment with an agent for improving sugar contents (Patent Literature 3 to 5), red light irradiation treatment after the end of the light period (Patent Literature 6), and the like are also known as techniques for achieving high sugar contents of tomatoes. However, conventional techniques for achieving high sugar contents require specialized cultivation technology and cultivation facilities, and therefore, there are still many problems in terms of economic efficiency, labor efficiency, and stability. Development of a new technology for achieving high sugar contents of tomatoes with decreased labor and cost has been awaited.
Meanwhile, although tomatoes are self-pollinating plants, it is known that the greenhouse cultivation of tomato results in reduced pollination and fruit-setting rates due to lack of wind and insects which assist pollination. Therefore, methods for promoting parthenocarpy and fruit enlargement by plant hormone treatment of flower trusses are widely used. Alternatively, methods for promoting pollination using bumblebees or vibrators are also widely used. However, plant hormone treatment and treatment for promoting pollination using vibrators require a lot of labor, which results in a significant decrease in labor efficiency. Although labor efficiency of a method using bumblebees is good, the method is problematic in causing an increase in cost and effort for temperature control in a facility in the summer and winter, due to the limitation of the temperature range for activity of bumblebees. In addition, reduction of pollen fertility in summer and winter makes it difficult to secure stable fruit production throughout a year in the case of fruit-setting via pollination/fertilization, which is also problematic. Therefore, to realize stable cultivation with decreased labor and cost while reducing influence of environmental factors such as seasonal factors, there is a demand for development of technology for inducing parthenocarpy in tomato plants at improved work efficiency.
Relatively new techniques for inducing parthenocarpy of tomato plants include a method using a non-plant hormonal fruit-setting promoter (e.g., Patent Literature 7) and a method for introducing a parthenocarpic gene into tomato plants (Patent Literature 8 to 9). However, the method using a fruit-setting promoter is still problematic in terms of labor efficiency. In addition, a parthenocarpic tomato variety that is produced using a conventional parthenocarpic gene is problematic in terms of fruit quality due to softening or the like. In view of the above, development of a tomato variety having parthenocarpy and preferable fruit characteristics has been awaited.
Further, development of a parthenocarpic induction method that allows stable fruit production with labor-saving and low-cost has been awaited not only for tomato plants but also for many of other cultivation plants.